Although the man-made trans-unsaturated fats are generally detrimental to human health, a new class of naturally occurring trans fatty acids, called conjugated linoleic acids (CLA), which are found in dairy products and meat, have been shown to have several health benefits. The dietary CLA, which are composed of many isomers, have been reported to be protective against several diseases including cancer, obesity, heart disease, inflammation and diabetes. However, these results are controversial, and in addition to the beneficial effects, many undesirable effects, including fatty liver, lipodystrophy, and insulin resistance have also been reported. The mechanisms underlying the wide range of CLA effects are unknown, but the two major isomers in the diet, namely cis 9 trans 11 (c9t11), and trans 10 cis 12 (t10c12) CLA have markedly different biological effects, with the former being associated with the anti-cancer effects, and the latter with the anti-obesity effects. Although the differential effects of CLA isomers on expression of specific genes may explain some of the results, there is neither an unifying hypothesis to explain the vast number of seemingly unrelated effects of CLA, nor a physicochemical basis for their isomer-specific effects. To address these issues, we propose two novel hypotheses. In Specific Aim 1, we will use cultured hepatocytes and adipocytes to test the hypothesis that the long term effects of CLA are the result of modification of the composition and function of membrane rafts, which play critical roles in numerous cellular functions. In Specific Aim 2, we will address the hypothesis that the cis double bond at position 9 of the fatty acid chain plays a critical role in determining the physicochemical properties of the membranes, including fluidity, cholesterol-binding, and permeability, and that this is responsible for the differential biological effects of the c9t11 and t10c12 CLA isomers. In addition to the physicochemical studies with the synthetic phospholipids, we will employ molecular dynamics simulation to study the atomic level interaction of CLA with other membrane components, to test this hypothesis. PUBLIC HEALTH RELEVANCE: Conjugated linoleic acids (CLA) are popular nutritional supplements, which are consumed because of their reported protection against several diseases including cancer, obesity, heart disease and inflammation. The results obtained from these studies should enable us not only to understand the mechanism of action of CLA, but also to modify their structures for optimum beneficial effects without the undesirable side effects such as insulin resistance.